[U-Boot] [PATCH 0/5] dcache support for Raspberry Pi 1

[Albert ARIBAUD]

Hello Alexander,

On Mon, 06 Jul 2015 20:24:31 +0200, Alexander Stein
<alexanders83 at web.de> wrote:
> Hello Albert,
> 
> On Monday 06 July 2015, 09:39:40 wrote Albert ARIBAUD:
> > On Sat,  4 Jul 2015 11:48:39 +0200, Alexander Stein
> > <alexanders83 at web.de> wrote:
> > 
> > > dcache supprt increases the MMC read performance on RPI 1 from 5,4 MiB/s to
> > > 12.3 MiB/s. It doesn't seem to have any affect on RPI 2 though. I just get
> > > error messages about non-cacheline aligned address upon invalidation.
> > 
> > Could it be that code needed to support dcache is not the same for
> > rpi_2's bcm2836 than it is for rpi's bcm2835?
> 
> Sure, bcm2835 is a armv6 while bcm2836 is a armv7.
> 
> > Anyway: if code properly handles unaligned addresses then it should not
> > throw an error message about it. Can you look into why the error is
> > thrown?
> 
> Apparently it does not handle non-cacheline aligned addresses transparently or silently.
> 
> Here is the part of the code:
> > static void v7_dcache_inval_range(u32 start, u32 stop, u32 line_len)
> > {
> > 	/*
> > 	 * If start address is not aligned to cache-line do not
> > 	 * invalidate the first cache-line
> > 	 */
> > 	if (start & (line_len - 1)) {
> > 		printf("ERROR: %s - start address is not aligned - 0x%08x\n",
> > 			__func__, start);
> > 		/* move to next cache line */
> > 		start = (start + line_len - 1) & ~(line_len - 1);
> > 	}
> 
> I don't know why (a) the cache invalidation is only done from the next cache line and (b) why this can't be done transparently without printing an error.
> But currently I'm not keen on fiddling with armv7 caches.

Well, I can see why.

Let's assume were invalidating the second half of a cache line because
that's where a buffer starts which we want to force-read from external
memory because some device fille dthis buffer with important data.

Now, most probably the compiler and linker will have used the addresses
before our buffer to map some variables which may be unrelated to the
buffer.

At the time we're told to invalidate the buffer, these variables may be
modified in-cache but not yet written out to external memory. If we
invalidate the first cache line, then we erase these modifications --
they're lost.

Now, this is an unsolvable problem -- we can't flush these variables
before invalidating, because then we would flush the whole cache line,
which would overwrite and trash the buffer in external memory.

So anyway, we're doomed; there is nothing we can do -- hence the ERROR
message. From the on, we can either just give up and go hang(), or we
can try to save whatever can be, skip the half cache line and start
invalidating at the next boundary.

(same goes for the last address: it has to be at the end of a cache
line, or else we can neither invalidate nor flush.)

> Best regards,
> Alexander

Amicalement,
-- 
Albert.